Inside assisted GPS: helping GPS help you

Ever wonder how your mobile phone can find your location quickly, even on a …

Location, location, location

The Global Positioning Satellite (GPS) system can help you find yourself, or the nearest drycleaner, or a missing Christmas manger, or a hidden geocache. But our expectations of GPS are that it works accurately, instantly, and reliably on all our devices. That's asking a bit much from what is fundamentally 1990s-era technology.

Fortunately, there's help. Assisted GPS (AGPS) and a variety of complements and supplements to GPS can shrink the wait for a positive fix on a location— the Time To First Fix or TTFF—from multiple minutes down to as little as under a second without sacrificing accuracy.

Reducing that wait means that your pictures are instantly tagged with coordinates (geotagged), a map drops a pin on your current location before the map itself finishes loading, and a device you're using in a remote location knows right away that it's in the middle of nowhere. Oh, and Google and others can give you the right ads for your location without any tedious waiting on their part, either.

Typically, AGPS systems help a GPS figure out where the satellite signals that the receiver is picking up are located precisely at that moment. Related systems that aren't technically AGPS use a variety of means to combine with, replace, or enhance GPS data into providing a geographic result.

In this article, I'll explain how AGPS and alternatives work, and I'll conclude with a discussion of what's coming in the future for even more precise or esoteric location finding.

GPS basics

The GPS system, run by the US Department of Defense through an Air Force space division, uses a constellation of 32 satellites that each orbit the earth twice per day. A GPS receiver should be able to get signals from about 10 satellites at a time in ideal circumstances, but far fewer can be picked up reliably in most real-world conditions.

All the satellites constantly transmit data—the navigation message—over the same set of frequencies, using an encoding that allows 50 bits per second (really! 50 bps!) for a total of 1,500 bits of data to be demodulated from each satellite each 30 seconds.

On every minute and half-minute, each satellite transmits its notion of the precise time and its health, followed by its location and a path in orbit that's valid for as long as four hours (the ephemeris, pluralized ephermerides). It also transmits a subset of data about the other satellites in orbit, including a rougher position (the almanac). It takes 25 navigation messages, all received perfectly over 12.5 minutes, to assemble a full almanac. A timestamp is also included as part of each 300-bit (six-second) segment or sub-frame of the message.

With the timing information and the ephemerides from four satellites, a GPS receiver can perform trilateration, which allows a point to be plotted accurately to within about 5 to 15 meters (15 to 45 feet). Although geometrically only three satellites are needed, atmospheric effects and other issues introduce small errors in timing. A fourth satellite corrects those errors and allows an accurate and corrected time and elevation as well. (Some techniques for assisting or supplementing GPS can avoid the need for a fourth satellite, or even use fragmentary data from two satellites.)

A cold start

From a cold start with typically older GPS receivers—where the receiver has never been turned on, has been off for several weeks, has lost a battery charge, or has been moved a few hundred miles since its last activation—the entire almanac has to be retrieved over 12.5 minutes. And that's outdoors with good overhead visibility. Jean-Michel Rousseau, a staff product manager in the group at Qualcomm that handles GPS technology, explained that that's "the maximum time that the user would get a position assuming it had absolutely no knowledge of the GPS constellation."

While you can still buy certain kinds of standalone GPS devices with this lag—like the ATP Photo Finder used for geotagging digital camera images—most modern gear uses techniques to have enough information to avoid this cold start problem.

Most chipmakers and manufacturers of standalone GPS hardware now claim about 30 to 60 seconds for a warm start, with a few exceptions. "GPS receivers ship with some default almanac data in memory; as a result a receiver no longer has to decode this data off the satellites," Rousseau said. Qualcomm's gpsOne, for instance, can fire up in 35 seconds in a device that works in standalone mode, with none of the assistance available that we're about to learn about.